Natural antimicrobial effect of donkey milk using screening microbiological tests

Abstract

Donkey milk has been used since ancient times and is well known for its many beneficial properties. The aim of this study was to confirm the antimicrobial activity of donkey milk using a commercial screening assay and standard microbiological methods. The antimicrobial activity of donkey milk, along with cow, sheep, and goat milks, was initially tested by Delvotest® SP NT, a test based on the rapid growth and acid production of Bacillus stearothermophilus. This test showed false positive results to antibiotics in donkey milk, while all other milk samples were negative. This indicates the presence of natural antimicrobial agents in donkey milk. Further, the antimicrobial activity of all milk samples was evaluated using agar diffusion assay, against Staph ylococcus aureus (ATCC 25923), Bacillus subtilis subsp. spizizenii (ATCC 6633), Listeria monocytogenes 4e (ATCC 19118), Kocuria rhizophila (ATCC 9341), Escherichia coli (ATCC 25922) and Salmonella Typhimuri um (ATCC 14028). Only donkey milk showed small inhibition zones of Bacillus subtilis subsp. spizizenii and Salmonella Typhimurium at pH 6 and 8, respectively. To check the ability of foodborne pathogens to survive and grow in milk, all milk samples (donkey, cow, sheep, and goat milks) were contaminated at two levels ( 10 CFU/mL and 400 CFU/mL) with Salmonella Typhimurium and stored under different conditions (at 37 ? for 4 hours and at 4 ? for 24 hours). The highest number of Salmonella Typhimurium cells was recorded in cow milk, regardless of storage conditions. In this study, donkey milk demonstrated significant antibacterial activity against Salmonella Typhimurium in all storage conditions, which may be attributed to its naturally high content of lysozyme and lactoferrin.